To determine the speed of rotating elements coupled to an engine, for example an internal combustion (IC) engine, in order to properly time ignition sparks thereof, it has been proposed to provide inductive sensors which are magnetically coupled with the teeth of the starter gear coupled to the internal combustion engine. Wheel anti-block systems or vehicle anti-skid systems also utilize sensors which determine the rotary speed of the wheels. It has been proposed to connect special toothed or geared wheels or gear rings to the wheels of automotive vehicles, and to couple the teeth with inductive sensors.
Systems of this kind operate satisfactorily if the relationship between the gear teeth and the inductive sensors remains constant. Improper or inaccurate mounting, for example off-center positioning of the inductive transducers or the gear wheels, jolts, shocks or vibration due to uneven road surfaces or the like impair the relationship sought by the designer. The uniformity of the air gap between the inductive pick-up and the teeth of the gear with which it is coupled thus may vary. This variation typically is periodic. In some pick-ups, particularly in the customary inductive pick-ups, the speed signal can thus be falsified or altered. It is possible to avoid such errors by using differential sensors which are so located with respect to the circumferential direction of the gear teeth that they are responsive to the difference in magnetic field between two adjacent points, rather than two absolute magnetic fields.